Fuel cell systems usually include a hydrogen generation apparatus that includes a reformer, because an infrastructure for supplying hydrogen as a fuel has not been developed. The hydrogen generation apparatus includes a combustor. The combustor is configured to combust, for example, a fuel that has not been used by the fuel cell, thereby heating the reformer. Among such fuel cell systems, there is a known fuel cell system that includes a carbon monoxide sensor configured to detect the concentration of carbon monoxide contained in exhaust gas discharged from a combustor (see Patent Literature 1, for example).
It has been known that the carbon monoxide sensor included in the fuel cell system disclosed in Patent Literature 1 is tested for its detection sensitivity by using a test gas that contains carbon monoxide at a known concentration (see Patent Literature 2 and Patent Literature 3, for example).